1. Field of the Invention
The present invention relates to a headbox of a paper machine that includes a common feed line for a pulp slurry, a plurality of feed line sections into which the common feed line branches, and a device for influencing the composition of the pulp slurry flows in each of the feed line sections. The present invention also relates to a control device for a headbox of a paper machine wherein a cross-sectional profile sensor captures the weight basis of a paper web, and the control device controls the cross-sectional profile of the web and includes a device for influencing the composition of the pulp slurry in each of a plurality of feed line sections. The present invention also relates to a process for controlling a headbox of a paper machine which includes determining the weight basis cross-sectional profile of a paper web outside the headbox at a sensing rate, comparing this cross-sectional profile with a set point, and actuating a final control element in each section of the headbox in the event of variation between the actual value and the set point.
2. Discussion of Background Information
In the current state of the art, as shown by German Patent DE 43 16 054, a headbox of a paper machine is known which, at least in part, is formed of sections, enabling one to sectionally influence a feed pulp slurry. In headboxes of this type a weight basis cross-sectional control adjustment can occur, such that the pulp density is changed by section, while the fiber orientation cross-sectional profile is adjusted by sectionally altering the volumetric flows. The metered addition of diluting fluid generally is regulated by a control device. The actual cross-sectional profile of the pulp stream is thus determined outside the headbox, for example, in the screen section of the paper machine at certain time intervals .DELTA.T, and the actual cross-sectional profile is compared with set points stored in a control device. As a result of fluctuations between the actual and desired profiles, the pulp consistency is adjusted to the desired value or set point by actuating the control elements in the individual sections of the headbox. Headboxes, which operate with this type of cross-sectional profile control are shown, for example, in German Patent DE 42 39 845, the disclosure of which is hereby incorporated by reference.
However, the cross-sectional quality which can be achieved with such weight basis cross-sectional adjustment controllers is limited, in particular due to the longitudinal weight basis fluctuations occurring in certain processing systems. A primary cause for such longitudinal weight basis fluctuations is the occurrence of consistency fluctuations in the pulp stream. The main causes for consistency fluctuations in the pulp stream, in turn, are variations in thick matter and water in the screen, as well as uneven mixing of thick matter and water in the screen. De-mixing processes can also occur in the feed lines due to surface wall abrasion. It is also possible that interference in the control circuit, such as an actuator hysteresis, as well as measuring noise and regulator parameterization, can result in fluctuating metered quantities of thick material. Additionally, consistency fluctuations can occur in the headbox due to transient effects after a grade change, or after start-up of the processing system.
With previously known processes for controlling pulp stream consistency, the poor quality of measuring signals of the weight basis longitudinal profile were unable to suppress such consistency fluctuations. Moreover, the unavoidable weight basis longitudinal and residual profile variations result in a type of process noise, when measuring the actual cross-sectional profile. In order to accurately measure values for cross-sectional profile control, the process noise has to be suppressed by filtering the measurements. Due to this filtering, however, actuation of the cross-sectional control only occurs at large time intervals and/or with an inadequately low amplitude. As a result, the transient response of the control system slows down, and the cross section, which is adjustable by means of the control system, is limited by the random portion in the cross-sectional profile due to noisy profile data.